Crystal structure of bis(β-alaninium) tetrabromidoplumbate

This work focused on characterizing the structure from a stereochemical point of view, discussing the PbII character. The XRD data were collected at 200 K.


Chemical context
As plumbiferous compounds are commonly toxic, they are unfavorable for photovoltaic devices.Nonetheless, they have other important applications such as white-light-emitting materials (Peng et al., 2018), luminescent sensing (Wang et al., 2019;Wang, 2020;Martı ´nez-Casado et al., 2012), ferroelectric materials (Gao et al., 2017), non-linear optical materials (Chen et al., 2020) and semiconductors (Terpstra et al., 1997).Leadcontaining materials also are attractive from a stereochemical point of view.The Pb 2+ ion has a 6s 2 electron pair, which is crucial for the stereochemistry of Pb II .When the 6s 2 electron pair takes part in hybridization between the s and p orbitals, the lead atom is stereochemically active and has a hemidirected coordination, otherwise the lead atom exhibits a regular coordination sphere (Casas et al., 2006;Seth et al., 2018).
Our research group has been studying various amino acid salts for a long time (Fleck & Petrosyan, 2014), and we assumed that amino acids could also be used to synthesize organic-inorganic hybrid materials.After the successful synthesis of (GlyH)PbBr 3 (Tonoyan et al., 2024), efforts were focused on obtaining (GlyH)PbI 3 and related phases.Later, it was attempted to synthesize salts of �-alanine in the same manner; however, instead of (�-AlaH)PbBr 3 , crystals of (�-AlaH) 2 PbBr 4 were formed.

Structural commentary
The title compound (�-AlaH) 2 PbBr 4 crystallizes in the monoclinic space group P2 1 /n.The asymmetric unit contains one formula unit.The molecular arrangement is shown in Fig. 1.As can be seen from the dihedral angles (Table 1), both �-alaninium cations have the most common gauche conformation (Fleck et al., 2012).
The Pb 2+ centers of the anion exhibit a holodirected sixcoordination with an octahedral geometry.Therefore, for neighboring bromine atoms, the Br-Pb-Br angles are close to right angles, varying from 80.90 (1) to 103.14 (1) � (Table 1).The lead atom forms three partial covalent bonds Pb1-Br2, Pb1-Br3, and Pb1-Br4, and also three coordination bonds with partial covalent character Pb1-Br1, Pb1-Br2 i and Pb1-Br4 ii (Table 1).Despite the range of Pb1-Br distances, the average value of 3.0259 A ˚is close to the average value of 3.0310 A ˚in PbBr 6 octahedra, regardless of the anion, for 284 structures in the Cambridge Structural Database (CSD2023.2.0, version 5.45, November update; Groom et al., 2016).The PbBr 6 octahedra form a 2D structure with four shared vertices: Br2, Br2 i , Br4, and Br4 ii (Fig. 2).The octahedra share only vertices, not edges nor faces.The two terminal opposite atoms Br1 and Br3 are located on the surfaces of the layer and the octahedra are arranged in such a way that the angles of the Pb-Br-Pb bridges are close to linear (Table 1), which leads to square-shaped voids between the octahedra.

Supramolecular features
The packing in the crystal together with the hydrogen-bond network is shown in Fig. 3.The anionic layers are parallel to the (001) plane, with an interlayer distance of 11.026 (1) A ˚.The �-alaninium cations are positioned between the anionic layers with the amino and carboxyl groups oriented towards Figure 2 2D structure of the PbBr 4 anion viewed along the c axis.Part of the anion is shown in an octahedral style.Symmetry codes: ( those layers.The �-alaninium cations cross-link neighboring layers of anions through hydrogen bonds between terminal bromine atoms and NH 3 + , and OH groups (Table 2).Each carboxyl group forms one O-H� � �Br hydrogen bond, while the ammonium groups form two and three N-H� � �Br hydrogen bonds.Intramolecular N1A-H11A� � �O2A and N1B-H12B� � �O2B hydrogen bonds are present in the �-alaninium moieties (Table 2).

Database survey
A survey of the Cambridge Structural Database (CSD2023.2.0, version 5.45, November update; Groom et al., 2016) revealed 320 structures containing PbBr 4 .There were 91 duplicate structures solved at different temperatures, and several inappropriate structures; thus, overall 224 structures were considered.Among them, the title compound was found with refcode YINFIO (Zu et al., 2023), determined at room temperature.
A 1D structure anion may consist of either PbBr 5 squarepyramids (3 structures) or PbBr 6 octahedra (16 structures).The square-pyramids are alternately connected by a shared bromine atom, with three bromine atoms remaining terminal.Chains can be linear (Fig. 4c: RUSBUF, Lv et al., 2020) or zigzag (Fig. 4d: SOHYAS, Li et al., 2019).Octahedral PbBr 6 monomers can attach two, three or four adjacent octahedra, have four or three shared bromine atoms, and two or three terminal atoms.Chains can be linear (Fig. 4e When each monomer has four adjacent monomers attached, and each pair of adjacent monomers shares one bromine atom, a 2D structure is formed (195 structures).Each lead atom has two terminal bromine atoms in the 2D structure of the anion.There are two main options, depending on the terminal atoms.When terminal atoms are trans positioned, a planar arrangement of octahedra is formed.In our case, the Pb-Br-Pb angles are close to linear (Table 1, Fig. 2).An ideal form of this is a rare centrosymmetric anion in the structure of COJKIZ01 (Long et al., 2024) with 180 � Pb-Br-Pb angles, and square-shaped voids between the octahedra.There are 36 structures with square or near square voids, and this is the second most common geometry at 16%.In other cases, the Pb-Br-Pb angles differ from 180 � , and values down to 139 � can be encountered, causing rhombic voids (Fig. 4i

Figure 4
The (PbBr 4 ) 2À anion geometries.Note that one more 2D form is missing here as it is shown in Fig. 2. The numbers of CCD structures for a given type of anion are indicated.

Synthesis and crystallization
As initial reagents, we used amino acid �-alanine (99% NT) and hydrobromic acid (48%) purchased from Sigma-Aldrich and lead (reactive grade).Initially, an excess volume of hydrobromic acid was added to a preliminary weighted amount of lead.When the reaction between them was completed (when no H 2 gas is released), the unreacted lead was removed by filtration, dried and weighed.The quantities of lead(II) bromide (PbBr 2 ) obtained and unreacted acid (HBr) in the filtrate were calculated.The appropriate amount of �-alanine was added to it and mixed to achieve a final solution with a 1:1:6 molar ratio of �-Ala, PbBr 2 and HBr, respectively.Instead of the desired compound (�-AlaH)-PbBr 3 , (�-AlaH) 2 PbBr 4 was obtained as yellow crystals (Fig. 5).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were treated as riding on their parent atoms [C-H = 0.99 A ˚, N-H = 0.91 A ˚; U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (N)] except those of the carboxyl groups, which were refined with the restraint U iso (H) = 1.5U eq (C).

Special details
Experimental.A selected fragment of a crystal was mounted on a MiTeGen loop with silicone grease and examined by single crystal X-ray diffraction at 200 K on a Bruker APEX II diffractometer equipped with a CCD area detector, an Incoatec Microfocus Source IµS (30 W, multilayer mirror, Mo-K α ) and an Oxford Cryosystems Cryostream 800 Plus LT device.Several sets of phi-and omega-scans with 2° scanwidth were combined at a crystal-detector distances of 40 mm to achieve respective full sphere data up to 65° 2θ.Data handling with integration and absorption correction by evaluation of multi-scans was done with the Bruker Apex5 suite (Bruker, 2024).The structure was solved by direct methods (Sheldrick, 2015a); subsequent difference Fourier syntheses and least-squares refinements yielded the positions of the remaining atoms using the SHELX software (Sheldrick, 2015b) implemented in the ShelXle GUI tool (Hübschle et al. 2011).Non-hydrogen atoms were refined with independent anisotropic displacement parameters.Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes. Fractional

Figure 3
Figure 3Packing diagram of the structure of (�-AlaH) 2 PbBr 4 viewed along the a axis.Hydrogen bonds are shown as dotted lines.

Table 3
Experimental details.